Specifying the Appropriate Background

There are several potential types of background and noise that can affect the observations; the main contributors being:

  1. Earthshine

  2. Zodiacal light

  3. Geo-coronal emission Lines (UV)

  4. Atmospheric helium (IR)

  5. Thermal background (IR)

  6. Dark current

  7. Read noise (CCDs and NICMOS)

The sum of the External Background components is presented on the Results page. Note that each of the components of External Background can be switched off (set to None or No airglow) so that one of the components can be selected for display on the Results page. All components should be included when simulating observations, as artificially turning off the sky does not represent actual observing conditions.

External Background

Earthshine

Earthshine can vary strongly depending on the limb angle, on the fraction of the earth that is sun-lit, and on the fraction of cloud cover. At low limb angles, it can dominate the zodiacal light, which varies by only a factor of ~3 throughout the sky available to HST (See Giavalisco et al. WFC3 ISR 2002-12).

The table below lists the relative levels of Earthshine selectable in the ETC.

Table 1: Earthshine Background Contributions Relative to “High”

Level:

None

Average

High

Extremely High

Earthshine

0%

50%

100%

200%

Limb angle

N/A

50 deg

38 deg

24 deg

The phase II special requirement LOW-SKY ensures that the limb angle will be greater than 40 degrees, i.e. that it will avoid the limb angle of the High Earthshine case by at least 2 deg. This requirement greatly reduces scheduling opportunities, so should be requested only when critical to the science and should be well justified in the phase I proposal.

Zodiacal Light

The Zodiacal light values given in Table 2 are in ecliptic co-ordinates \beta (ecliptic latitude) and \lambda-\lambda_{\odot} (helio-ecliptic longitude or elongation). See the following figure.

Zodiacal Light Angle Illustration

When a user request to calculate the contribution of the Zodiacal light with position, first the Equatorial coordinates are converted to Ecliptic coordinates. The relation between these set of coordinates is:

\sin(\beta) = \sin(\delta) \cos(\epsilon) - \cos(\delta) \sin(\epsilon) \sin(\alpha) \ \ (i) \\ \cos(\lambda) = \cos(\alpha) \cos(\delta) / \cos(\beta) \ \ (ii) \\ \sin(\lambda) = [\sin(\delta) \sin(\epsilon) + \cos(\delta) \sin(\alpha) \cos(\epsilon) ] / \cos(\beta) \ \ (iii)

or

\sin(\beta) = \sin(\delta) \cos(\epsilon) - \cos(\delta) \sin(\epsilon) \sin(\alpha) \ \ (iv) \\ \tan(\lambda) = [\tan(\delta) \sin(\epsilon) + \cos(\epsilon) \sin(\alpha) ] / \cos(\alpha) \ \ (v)

where:
\alpha = right ascension
\delta = declination
\epsilon = obliquity of the ecliptic with \epsilon = 23.446^o for equinox 1950.

Since we know \epsilon we can then use (i) and (ii) to calculate \beta and \lambda.

To solve for the ambiguity of the sign you should calculate the equations (iii) and (v) and

  • if \lambda_1 < 0 and \lambda_2 > 0 and \lambda_3 > 0 then \lambda = \lambda_1 + 180

  • if \lambda_1 > 0 and \lambda_2 < 0 and \lambda_3 > 0 then \lambda = \lambda_1 + 180

  • if \lambda_1 < 0 and \lambda_2 < 0 and \lambda_3 > 0 then \lambda = \lambda_1 + 360

where:
\lambda_1 = \tan^{-1}(\tan(\lambda))
\lambda_2 = \sin^{-1}(\sin(\lambda))
\lambda_3 = \cos^{-1}(\cos(\lambda))

The ETC then calculates the helio-ecliptic logitude or elongation (sun angle between sun-earth and earth particle) to determine the brightness along the line of sight:

\lambda' = \lambda - \lambda_{\odot}

where:
\lambda_{\odot} is the equatorial longitude of the Earth

\lambda_{\odot} is derived as follows:

\lambda_{\odot} = 360 \times \Delta / 365.25 \ \ (vi)

where:
\Delta is the number of days from the Vernal Equinox of the selected year (March 20).

Once the Helioeclipic coordinates are calculated, the ETC performs a bilinear interpolation using the values in Table 2 in order to derive the Johnson V magnitude for the Zodiacal contribution. The result is then fed to the pysynphot task renorm to renormalize the Zodiacal light spectrum model (currently zodiacal_model_001.fits) to the derived V magnitude.

Table 2 provides the approximate values of the Zodiacal light background (V_{mag} / arcsec^2) in ecliptic latitude (\beta) and helio-ecliptic longitude (\lambda' = \lambda - \lambda_{\odot}). The SE values correspond to the solar exclusion zone. This table has been updated for cycle 24 using a finer grid to provide better estimates of the zodiacal light as a target approaches the 50 degree HST-to-target limiting angle. It was again modified for ETC 29.1 to correct typos in a few positions (marked with ‘*’).

Table 2: Zodiacal Light Background Contributions

\lambda'

(degrees)

\beta (degrees)

0

5

10

15

20

25

30

35

45

50

60

75

90

0

SE

SE

SE

SE

SE

SE

SE

SE

SE

22.0708

22.5136

22.9538

23.2298

5

SE

SE

SE

SE

SE

SE

SE

SE

SE

22.0816

22.5136

22.9538

23.2298

10

SE

SE

SE

SE

SE

SE

SE

SE

SE

22.1033

22.5210

22.9538

23.2298

15

SE

SE

SE

SE

SE

SE

SE

SE

SE

22.1454

22.5360

22.9538

23.2298

20

SE

SE

SE

SE

SE

SE

SE

SE

SE

22.2004

22.5743

22.9649

23.2298

25

SE

SE

SE

SE

SE

SE

SE

SE

22.0808

22.2586

22.6141

22.9762

23.2298

30

SE

SE

SE

SE

SE

SE

SE

SE

22.1578

22.3237

22.6554

23.0107

23.2298

35

SE

SE

SE

SE

SE

SE

SE

21.9203

22.2350

22.3924

22.7071

23.0224

23.2298

40

SE

SE

SE

SE

SE

SE

21.8257

22.0287

22.3181

22.4628

22.7522

23.0343

23.2298

45

SE

SE

21.0810

21.3356

21.5717

21.7872

21.9545

22.1379

22.3948

22.5232

22.7801

23.0707

23.2298

50

20.8432

21.0663

21.3194

21.5397

21.7408

21.9486

22.0833

22.2472

22.4715

22.5837

22.8080

23.1071

23.2298

60

21.1844

21.3356

21.5842

21.7872

21.9859

22.1525

22.2937

22.4437

22.6304

22.7237

22.9104

23.1212

23.2298

75

21.6258

21.6965

21.8737

22.0611

22.2180

22.3621

22.4989

22.6319

22.7801

22.8542

23.0024

23.1607

23.2298

90

21.9155

21.9768

22.0660

22.2350

22.3948

22.5284

22.6470

22.7699

22.9104

22.9807

23.1212

23.2016

23.2298

105

22.1315

22.1419

22.2124

22.3686

22.5136

22.6387

22.7614

22.8912

22.9990

23.0529

23.1607

23.2298*

23.2298

120

22.2639

22.2757

22.3305

22.4844

22.5980

22.7071

22.8186

22.9646

23.0707

23.1237

23.2298

23.2736

23.2298

135

22.3181

22.3243

22.3948

22.5284

22.6304

22.7339

22.8483

22.9224*

23.0707

23.1237*

23.2298*

23.2885

23.2298

150

22.3181

22.3243

22.4014

22.5210

22.6060

22.6896

22.7801

22.8639

22.9990

23.0665

23.2016

23.3037

23.2298

165

22.2180

22.2407

22.3181

22.4014

22.4989

22.5743

22.6554*

22.7435

22.9104

22.9938

23.1607

23.3037

23.2298

180

22.0418

22.1315

22.2236

22.3243

22.4216

22.5210

22.6304

22.7348

22.8998

22.9823

23.1473

23.3037

23.2298

Derived from Leinert, et al. (1998), Table 17.

This is used to normalize the Zodiacal Light spectra used by the ETC and shown in the figure below.

Zodiacal Light Template

Geo-Coronal Emission Lines

Table 3: Geo-Coronal Emission Line Properties

Line

Low

Average

High

Wavelength Angstrom

Flux(erg cm^-2 s^-1)

FWHM Angstrom

Flux(erg cm^-2 s^-1)

FWHM Angstrom

Flux(erg cm^-2 s^-1)

FWHM Angstrom

Lyman_Alpha

1215.7

6.1e-14

0.04

3.05e-13

0.04

6.1e-13

0.04

O_I

1302

3.8e-16

0.013

2.85e-14

0.013

5.7e-14

0.013

O_I

1356

3.0e-17

0.013

2.5e-15

0.013

5.0e-15

0.013

O_II

2471

1.5e-17

0.023

1.5e-15

0.023

3.0e-15

0.023

The strength of the geo-coronal Lyman alpha varies between about 2 and 20 kilo Rayleighs, depending on the time of observations and the position of the target relative to the Sun, and can be kept low by the special requirement “SHADOW”. For more details, see the corresponding Instrument Handbook.

Atmospheric Helium

Short-wavelength WFC3/IR imaging filters (F105W and F110W) and WFC3/IR grisms (G102 and G141) are subject to excess background emission caused by metastable Helium atoms in the solar-illuminated upper atmosphere. The background is due to He I emission at 10830 Angstrom. Its contribution is negligible in Earth’s shadow but increases sharply when the telescope is outside of shadow (up to 5 e-/s/pix in extreme cases). The He background also increases with decreasing limb angle, but can still be significant as high as 40 degrees above the Earth limb. For more information on this background component, see WFC3/ISR 2014-03 (Brammer et al.)

The contribution of atmospheric helium to the WFC3 background is negligible in Earth’s shadow, corresponding to the “None” default ETC setting. The “Average”, “High” and “Very High” values implemented in the ETC correspond to 0.1, 0.5, and 1.5 e-/sec/pix in F105W and are the 50, 75, and 95 percentile fluxes of the excess flux, per exposure, from the Helium line determined from archival observations. The ETC model of this emission is a gaussian source at 10830 Angstrom with FWHM = 2 Angstrom.

Thermal Background and Noise

The thermal background is negligible below about 8000 Angstrom and increases slowly towards longer wavelengths. For WFC3/IR, the thermal count rate (per unbinned pixel) is calculated by an algorithm that is described in detail in “Thermal Background Limitations for IR Instrumentation Onboard HST”, Sosey, M., Wheeler, T., Sivaramakrishnan, A., 2003, NICMOS ISR 2003-007.

Detector dark current is an intrinsic source of background counts. The dark current rate is dependent upon the detector design and temperature. It is measured in counts per unbinned pixel per second. For CCDs, 1 count = 1 e-; for MAMAs, 1 count results from each charge cloud detected.

CCD detectors and the WFC3 IR detector are subject to noise caused by the process of reading out the charge accumulated by the pixels. The amount of read noise varies by detector and as a function of gain. CCD read noise is measured per binned pixel, per read. WFC3/IR read noise per pixel is measured by a linear fit to a complete sequence of 15 reads. Fewer reads (smaller NSAMP) result in greater read noise, not taken into account in the computation. (See WFC3 ISR 2009-21).

Table 4: Dark Current and Read Noise Values

Instrument

Detector

Dark Current

Read Noise

(counts \cdot s-1 \cdot pixel-1)

(counts \cdot pixel-1)

ACS

HRC

0.058

4.7 (for gain=2.0)

SBC

8.52e-6

N/A

WFC

0.0161

4.45 (for gain=2.0)

COS

FUVB

7.00e-6 (target acquisition)

N/A

FUVA

6.34e-6 (target acquisition)

"

FUVB

4.15e-6 (spectroscopic)

"

FUVA

3.34e-6 (spectroscopic)

"

NUV

1.22e-3

N/A

STIS

CCD

0.026 (low; top of detector)

6.5 (for gain=1); 8.9 (for gain=4)

0.032 (medium; middle of detector)

"

0.038 (high; bottom of detector)

"

FUV

3.0e-5 (no glow component)

N/A

7.5e-5 (low glow component) [1]

"

1.5e-4 (medium glow component)

"

4.0e-4 (high glow component)

"

NUV

0.0013

N/A

WFC3

IR

0.02

14.6 (for gain=2.5)

UVIS1

0.00213 ADU/s/pix = 0.00319 counts/s/pix

3.0 (for gain=1.5)

UVIS2

0.00222 ADU/s/pix = 0.00333 counts/s/pix

3.0 (for gain=1.5)